Printing-telegraph exchange system



1,613,685 G. s. VERNAM ET AL PRINTING TELEGRAPH EXCHANGESYSTEM Filed Nov. 10, 1925 2 Sheets-Sheet Jan. 11 1927.

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Wharf 5 Verna/77 flaw/d fiFe/ry ill) Patented Jan. 11, 1927.

UNITED STATES ATEjNT OFFICE.

GILBERT S. VERNAM, OF RIVEREDG-E, NEW JERSEY, AND DONALD B. PERRY, OE YON- KERS, NEW YORK, ASSIGNORS TO AMERICAN TELEPHONE: AND TELEGRAPH COM- IPANY, A CORPORATION OF NEW YORK.

i PRINTING-TELEGRAPH EXCHANGE SYSTEM.

' Application filed November 10, 1923. Serial No. 674,010.

This invent-ion relates to printing telegraphy and particularly to machine switch ing printing telegraph exchange systems.

An objectof the invention is a machine switching telegraph system which is capable of extending the line of a printing telegraph subscriber over a telegraph line equipped with duplex apparatus to a printing'telegraph subscriber located in a distant city and which functions to extend the connections in either direction over said telegraph line between said subscribers.

In the preferred form, the invention contemplates the provision of a machine switching telegraph exchange system, which may be operated to selectively connect over a telegraph line equipped with duplex appa-- ratus, the lines of two printingtelegraph subscribers who may, after the completion of the connection, converse telegraphically. To facilitate and expedite the operation of the system, machine switching apparatus is designed to be capable of completing connections in either direction, thereby providing both incoming and outgoing service on the same line conductor. In addition to the above mentioned provision, following the completion of the connection by the machine switching apparatus, relay'means are operated for converting the system from full duplex to half duplex operation.

Referring to the drawings, the apparatus associated with one end of a telegraph line is shown when Figures 1 and 2 are'placed end for end with Fig. 2 atthe right of Fig 1. It is of course understood that similar equipment is associated with the other end of the line for completing the desired con.-

nections. Fig. 3 is a. schematic illustration of a system comprising a plurality of stations.

In the drawings, there is shown a printing telegraph subscribers station A which may be located, for example, in New York and whose line E may be connected to one of a plurality of telegraph lines 5 through, suitable machine switching appar'atussuch-as a line switch B, a selector switch C and a connector switch D. The arrangement of the apparatus in this manner makesit possible for a printing telegraph subscriber located in one city to automatically establish a telegraph connection over a telegraph line with "a subscriber located in a distant city.

Each of the printing teliag-raph subscribers stations is provided with an impulse sending device and printing telegraph equipment, the latter comprising the usual transmitter and receiving print-er for performing their respective functions.

A better understanding of the invention may be had from the following description when considered in conjunction with the accompanying drawings, wherein-one embodiment of the invention is illustrated.

Referring first to Fig. 3, there is illustrated a calling station A having a line leading to thecentral office and terminating in the line switch '13. The line switch B has access to trunks leading to the selector switch G, which in turn has access to outgoing telegraph lines extending to a distant central office and there terminating in connector switches such as the switch E. Connector switches E have access to local telegraph lines leading to stations such as the called station F. The connector switch D in the first office is also illustrated.

Assume for the purpose of illustration that the subscriber at station A located at New York desires to communicate telegraphically with a subscriber located in Pittsburgh. The subscriber; at station A desiring to establish such a connection would operate switch 9 which causes the energization of switching relay 10. The switch 91, in addition to causing the energization of the relay, 10, may also be utilized for con necting the power circuit with theprinter apparatus. For the sake of clearness, this part of the system has not been shown.

Switching relay 1O disconnects the call bell 11 from in bridge of the line conductors E and connects the transmitter, printer and impulse sending device 12 in seriestherewith. When the front contact of relay is closed, an energizing circuit is established forline-relay 15 of the line switch B. The circuit of this relay may be traced from grounded battery, through the. winding of relayj15, uppermost armature and contact of cut-01f relay 16, ring conductor of the line E, transmitter and printer in series contact 13 of the impulse sending device 12, armature and front contact of relay 10, lowermost contact and armature of cut-01f relay 16 to ground. The line relay 15 in closing its contact establishes an energizing circuit for cut-off relay 16, which opens at its con- 7 tacts, the energizing circuit of relay but inasmuch as this relay is slow-to-release, it

does not release its armature until the energization'of relay 26 associated with the selector switch O is energized for establishline E, the transmitter and printer in series.

contact 13 of the impulse sending device 12, armatureand front contact of relay 10, tip conductor of the line E, contact of the line switch B, upper middle armature and back contact of relay 32, and the winding of line relay 'to ground. Line relay 25 in energizing, establishes an energizing circuit for slow-to-release relay 26 extending from .ate the impulse sending device 12 fortransand back contacts.

grounded battery, through the winding of' relay 26, front contact and armature of-re-' lay 25, and the lowermost contact and armature of relay 32 to ground. Relay 26 in closing its right-hand front contact, conditions, an energlzing circuit for the series relay 28 and thevertical magnet 29. Relay 26 also, completes the locking circuit for the cutoflf relay 16 extending from grounded battery, through the upper winding of cut-off relay 16, contact 18 of the line switch B, and the left-hand armature and contact of relay 26 to ground. As previously stated, line relay 15 is of the slow-to-release type and remains operated after its energizing circult is opened at the uppermost contact and armature of cut-oif relay l6. and until relays.

25 and 26 have been energized to maintain cut-01f relay 16 energized, after which line relay 15 releases its armature; Cut-ofi re .lay 16 now deperlds for its energization on the maintenance of the ground connection at the left-hand armature of relay 26.

The subscriber at station A may now opermitting a .series of impulses characteristic of the first digit of the called subscribers line. This digit identifies the city in which the called subscriber is located. Due to the operation of the impulse sending device 12,

the contactthereof will be opened and closed. thereby causing the deenergization and energization of line relay 25 which thereupon vibrates its armature between its front On each closure of the back contact, an energizing circuit for the series relay 28 and the vertical magnet 29 is closed. This circuit may be traced from grounded battery, through switching eleincrease ment 30 which is operated only after the selector switch has t-aken ll rotary steps, winding of vertical magnet 29, winding of series relay28, right-hand front contact and armature of slow-to-release relay 26, back contact and armature of relay 25, and the lowermost contact and armature of relay 32 to ground.

The series relay 28 and the vertical magnet 29 are thereupon energized, the vertical magnet moving the wiper shaft, upon which the wipers. 33, 34 and 35 are mounted, to the level in the bank of contacts allotted to the particular digit dialed, as'for example, contacts 36, 37 and 38. On the first vertical step of the wiper shaft, off-normal contacts 41 and 42 are closed, thereby conditioning the energizing circuits, for the interrupter relay 43 and the release magnet 45.

Upon the energization of relay 28 which as previously described is energized at the same time as vertical magnet 29, the interrupter relay 43 willbe energized following the upward movement of the wiper shaftover a circuit which extends from grounded battery, through the winding of interrupter relay 43, cit-normal contact 42, armature and contact df relay 28 and the left-hand armature and contact of relay 26to ground. Following the closure of the left-hand contact of relay 43, a locking circuit is established therefor. extending from grounded battery, through the winding of interrupter relay 43, oft-normal contact 42, armature and contact of stepping magnet 44, lefthand contact and armature of relay 43 and the lowermost armature and contact of switching relay 32 to ground.

When the series of impulses ends, that is,

'impulsesproduced by the first operation of the impulse sending device 12, line relay 25 is again energized, thereby maintaining relay 26 energized, Owing to this operation, the series relay 28 is deenergized, closing the circuit of stepping magnet 44. The. energizing circuit of stepping magnet 44 be traced from grounded battery, through the winding of stepping magnet 44, right-hand contact and armature of interrupter relay 43, contact 49 and the left-hand armature and contact of relay 26 to ground. It is to be noted here, that the interrupter relay43 depends for its energization on the ground established at the lowermost armature and contact ofswitching relay 32, wh le the stepping magnet 44depends for its energization on the closure of the left-hand front contact of relay 26. The stepping magnet 44 now operates to move the wipers 33, 34 and 35 through onehorizontal step or in other words on to the first set of contacts. If the telegraph line which is connected to this setof contacts is free, there will be an absence of ground on the wiper 33. However, if the telegraph line happens to be busy, there will be a ground connected to this particular wiper. Assume the latter to be the case? This ground on the bank dontact 36 is now connected through Wiper 33, and an armature and back contact of relay 32, to the contacts of the stepping magnet 44 and the interrupter relay 43. Near the end of its stroke, the stepping magnet 44 opens its contact thereby openlng the locking circuit of interrupter relay 43: Accordngly, relay 43 is deenergized opening the energizing circuit of stepping magnet 44. Stepping magnet 44 now closes its back contact, closing an energizing circuit of interrupter relay 43 to the wiper 33 and the bank contact 36. At this time, interrupter relay 43 depends solely upon the wiper 33 for a circuit because its own circuit to ground at the 'relay 32 is opened. It Wiper 33 rests on a grounded contact, the interrupter relay 43 will again energize and again cause the energization of stepping magnet 44, caus'ng it to rotate the wipers to the next set of contacts. So long as the wiper 33 finds ground, the interrupter relay 43 and the stepping magnet 44 will vibrate.

During this time, the switching relay 32 I has had a ground on each end of its winding; therefore, it remains deenergized. When the wiper 33 engages an ungrounded contact, as for example 36,the interrupter relay 43 fails to energize. Switching relay- 32 immediately energizes over a circuit extending from grounded battery, through the winding of rela 43, switch member 31 which is operate only after the switch has taken ten rotary steps, and the left-hand armature and contact of relay 26 to ground. Due to the establishment of the c'rcuit just traced, switching relay 32 is energized but relay 43 remains inert. Switching relay 32 extends the sleeve conductor through the wiper 33, contact 36, conductor 54, right I outer armature and back contact of relay 67 to battery through the winding of relay 53 which operates and looks as hereinafter described. A locking circuit for switching relay 32 is established upon its operation extending from grounded battery, winding of relay 43, off normal contact 42, armature and contact ofmagnet 44. winding of relay 32, switch member 31, front contact and armature 39 of relay 32, wiper 33, contact 36, right outer armature and back contact of relay 67, locking contactsof relay 53, and the lowermost contact and armature of relay '55 to ground. The release of'switch- ,the widing and middle contact and armature thereof and the lowermost contact and armature of differential relay to ground, thereby connecting this ground with the sleeve conductor through the contact 36 and wiper 33 for maintaining cut-01f relay 16 and switching relay 32 energized independent of relay 26, which releases shortly after the deenergization of relay 25. I

If all of the telegraph lines connected to contacts corresponding to 37 and 38 in'the selected level are busy, the wiper 33 will be unable to find an ungrounded set of terminals. The wipers therefore will take 11 rotary steps. At this point in the operation, operating members 30 and 31 will be operated cutting off the switching relay 32 and the vertical magnet 29 and closing an energizing circuit for busy relay 46. The energizing circuit for busy relay 46 extends from grounded battery, operating member 30,. and front contact thereof, winding of'busy relay 46 and the conducting segments of the interrupter 48 to ground. The interrupter 48 is'provided with suitable conducting and insulating segments so that upon its rotation, characteristic impulses will be transmitted due to the energlzation and the deenergization of the busy relay 46 to cause the operation of the receiving printer for recording a symbol designating that the called line is busy. The energization, of busy relay 46 closes at its right-hand contact and armature an energizing circuit for release magnet 47 which controls the release of the interrupter 48. The interrupter 48 is of the start-stop type inasmuch as the printer equipment at the station A is also of this type, it being necessary in such systems to transmit a starting impulse for releasing the printermechanism, at the subscribers station.

If an idle telegraph line is seized, and. upon the closure of the two uppermost contacts of switching relay 32, a circuit is established for pole changer relay 56. This circuit may be traced from grounded battery, through the upper winding of the pole changer relay 56, upper winding of differential relay 55, contact 37, wiper 34, upper middle contactand armature of switching relay 32, contact 20 of the line switch B.

tip conductor of the line E, armature and front contact of relay 10, contact 13 of the impulse sending device 12, printer and transmitter in series, contact 19 of line switch B, uppermost armature and front contact of switching relay 32, wiper 35, contact 38. lower winding of differential relay 55, and

was

1 contact controlled by relay 60, an energizing circuit is established for control relay til extending from grounded battery, through the winding of control relay 61 and the contact and armature of relay 60 to ground. Control relay'tll establishes an energizing circuit for line relay 62 extending from grounded battery, through the upper contact and armature of relay 61, left-hand winding of line relay 62 and the uppermost armature and hack contact of relay til to ground. Line relay 62 is energized over the circuit just traced and'completes the energizing circuit for the release relay 67 which is of the slow-to-release type and which prepares an energizing circuit for the series relay 72 and the vertical magnet 73, the series relay 72 and the vertical magnet 73 which will be energized upon the closure of the back contact of line relay 62 in response to the further operation of the impulse send- ,i ng device 12 by the subscriber A. The release relay 67 also connects ground from. a back contact of relay 55, through one of its front contacts and the contact 36 which is multipled to corresponding contacts of other selector switches similar to C, thereby making this particular set'ot contacts busy and preventing the seizure of this line circuit by wipers which may be operated by another subscriber who desires to establish.

a connection over the telegraph line 5.

"When the subscriber A dials the tens digit,

pole changer relay 56 is operated in accord ance with such operation for transmitting impulses over the telegraph line 5 to operate receiving relay 60 located-atthe opposite end of this line, whereby line relay 62 .vi brat-es its armature and establishes alternately the circuit through the winding oil release relay 67 an through the contacts of the release relay 67 for the series relay 7 L and the vertical magnet 73. The release relay 67 "remains energized as does the series relay 72 but the vertical magnet 7 3 is quiclr acting and follows the impulses and lifts the wi.per shaft upon which the wipers 74, 75 and 76 are mounted. The series relay 7 2 operates on the first impulse and remains energized to the end of the series of impulses impressed on the vertical magnet 73. On the first upward movement of the wiper shattcontacts 70 and 71 are closed, while contact 69 is opened. The closure of contact 70 extends the energizing circuit of the vertical magnet 7 through the armature magnet 77. When the subscriber dials the" last digit, line relay 62. again vibratessending pulsations through the stepping relay 80 and the stepping magnet 77 in parallel. The energizing circuit of each of these devices may he traced from grounded battery, through their windings, contact and armature 88 of busy relay 89, back contact and armature of series relay 72, ofi-normal contact'70, right-hand middle front contact and armature of release relay 67, and the hack contact and armature of line relay 62 to ground. The stepping magnet 77 rotates the wipers 74, 75 and 76 over the bank of contacts in which contacts 83,84- and 85 are located. To these contacts, there are connected conductors 9a, 100 and 101, which extend to subscribers station A or similar stations, and for the purpose of illustration, it should he assumed that these conductors extend to another subscrihers station. No

interference with other connected lines is caused due to the movement of the wipers 74:, 75 and 76 over the back of contacts as the line wipers are opened at the contacts of switching relay 78.:

During the rotation of wipers 74, 7 5 and 76, the wiper a may pass over one ormore busy lines, the corresponding contacts of which aregrounded through the lower armature and contact of diil'e'rential relay 55 if busy as a result of an outgoing call or through the innermost upper armature and front contact otrelay 78 it lousy as a result of an incoming call. Busy relay 89 may be energized while passing over such contacts, but this will not interfere with the stepping magnet 77 as the circuit for this magnet 1s controlled through the contacts of the stepping relay'SO which remains energized an holds the circuit closed until the last series of impulses has been received. When this occurs, the stepping relay 80 will also be released and if the called for line is not husy, lousy relay 89 will be released due to the absence of ground on the contact to which the wiper 7a is now connected.

lit the called line is not busy, wiper 74 connects battery to the upper. winding of switching relay 7 8, which .is energized over a circuit extending from battery, upper winding of cut-old relay 16, conductor 96lcontact, 83, wiper 74, left-hand armature lid winding of stepping relay 78, extreme lefthand back contact and armature of busy relay 89, and the left-hand armature and front contact of release relay 67 to ground. Following the closure of the contacts of switching-relay 78,. a locking circuit therefor is established from grounded battery, throu h the lower winding and armature 97 an front contact thereof and the righthand innermost front contact and armature of relay 67 to ground. Ground is also connected to the wiper 74 due to the energization of the switching relay Z8 making the called line busy. Relay 78 also opens the circuit of the stepping magnet 77v and the bus "relay 89.

he wipers 75 and 76 are now closed through the upper Winding of the ringing cut-ofi' relay 99 and the source of ringing current 98. Ringing current will now be transmitted intermittently through the upper winding of ringing cut-ofi' relay 99, wiper 7 5,. contact 84, conductor 100, call bell 11, and associated condenser to ground. The call bell at the subscribers station will thus be operated for advising such subscriber that some one desires to communicate with him telegraphically and that his printer set should be connected with the line. The called for subscriber willthereupon actuate a switch corresponding to 9 causing the energization of the switching relay which connects the printerset with the line. The ringing cut off relay 99 is designed so that it will not operate on the'ringing current, but when the subscriber responds to the call by the operation of switching relay corresponding torelay 10, direct current will be permitted to flow through the upper winding of the ringing cut-01f relay 99 over the circuit traced for the ringing current except in place of passing through the call hell, it will pass through the transmitter, printer and contact 13 of the impulse sending device and back over the line to ground at the uppermost back contact of relay 99.- Ringing cut-ofi' relay 99 in operating, disconnects the source of ringing current from the line and establishes a locking circuit for itself extending from grounded battery,

through the lower winding, front contact and armature thereof and the right-hand innermost front contact and armature of re lease relay 67 to ground. It also removes the ground connection from the tip conductor and establishes a holding circuit for line relay '62 and an energizing circuitfor duplex relay 64. The circuits of these -relay's may be traced from grounded battery, through the connected winding, uppermost contact and armature of differential relay 55, contact and armature of ringing cut-01f relay 99 and the contact and armature 96 of relay'78 to ground. The'pursignals and the false release of the connector switch 1). Due to the energization of the duplex relay 64:, the ground connection on the lower winding of the differential relay is disconnected and the battery at the upper armature and contact of relay 61 is substituted therefor. When the called for subscriber responds by connecting the printer equipment with the line conductors individual to his station, the pole changer relay associated with the incoming end of the telegraph line 5 will be operated for transmitting over this line a marking impulse. This impulse, upon being received at the outgoing end of the telegraph line 5, operates receiving relay 60 which completes an energizing circuit for control relay 61 extending from grounded battery, winding of relay 61 and the contact and armature of relay 60 to ground. Relayfil'closes at its upper contact an energizing circuit for line relay 62 which may be traced from grounded battery, upper contact and armature of relay 61, left-hand winding of relay 62 and the uppermost armature and contact of duplex relay 64 to ground. Relay 53, associated with the outgoing end of the telegraph line 5, being at this point in the operation energized a circuit is completed for locking up relay 62 and causing the energization of duplex relay 64. The energizing circuit of duplex relay 64 may be traced from grounded battery, winding thereof right-hand front contact and armature of relay 53 and the front contact and armature of relay- 62 to ground. Upon the energization of the duplex' relay'64, a locking circuit therefore is established from grounded battery, winding thereof, armature 65 and front contact, and the extreme left-hand contact and armature of relay 53 to ground. Duplex relay 64 also disconnects the ground from its uppermost armature and connects battery thereto through the upper armature of control relay 61, whereby difierential relay 55 is prevented from operating. There is also established, uponthe energlzation of relay 64, a locking circuit for control relay 61.

A channel of communication is now established between the subscriber located at New York and the subscriber located at Pittsburgh and the transmission of messages between these two stations may now be commenced. In ma ny cases, however, it would first be necessary for theother subscriber located'at Pittsburgh to operate his transthe subscriber at Pittsburgh is in readiness to receive any messages which the sgbseriber" at New York may desirejo transmit.

Should the called for subscribers line be busy, busy relay 89 will be energized over a circuit completed through the wiper 74, and the contact 83 to which ground under these conditions will be connected. Busy relay 89 opens at its left-hand back contact and armature the energizing circuit of switching relay 78 thereby preventing the energization of this relay. Likewise, the energizing circuits of stepping relay 80 and stepping magnet 77 are opened when relay 8O releases after the last dial impulse and consequently, the stepping magnet cannot be actuated by any further dialing or printing impulses which might be sent from station There is also completed due to the energization of the busy relay 89, an energizing circuit for release magnet 90 ot the interrupter 91, which is ear to that described in connection with the interrupter 18 and which controls the o eration of the pole changer relay 56. n the operation of the interrupter 91, pole changer relay 56 is caused to move its armature in accordance with the segments arranged on the interrupter 91 for transmitting impulses over the telegraph line 5 which operate relay 60 and which control the operation of control relay, 61 for transmitting tothe calling station impulses characteristic of a symbol which designates that the line is busy. These impulses will efi'ect the 0 eration of the receiving printer at the ca 'ng subscribers station for designating to such subscriber that the called for line is busy and that thecall should be abandoned.

When the connection is completed, the release of the connector switch D is controlled jointly by both stations. 7 If the called station disconnects first, diderential relay will operate over the tin side of the line circuit. The operation of the difi'erential relay 55 opens the energizing circuit for the line relay 62 and are thereupon deenergized. When the duplexrelay ca releases, the line relay 62 will immediately operate again. The circuit is now completed it'rom gro und at the upper back contact and armature of relay 6% through the left-hand winding of relay 62 and the upper armature and contact of relay 61 to battery. The energization of relay 62 prevents the connector switch D from releasing until the calling station disconnects. When the calling station disconnects,

the pole changer relay 56 at that end of theline is caused to resume its normal position and accordingly transmits a spacing signal which causes the restoration of the receiving relay and the control relay 61 to normal. Relay 62 is thereby released and after a short interval, release relay 67 will be deenergized, thus causing the energization of release magnet 66 and the restoration of the connector switch D to its normal position.

the duplex relay 64: which relates lit the calling subscriber disconnects first, the selector switch will be released at the calling end and a' spacing impulse will be trans- .mitted as previously described to release the receiving relay and control relay 61. The line relay 62 and the duplex relay 6d will remain locked up through the upper contact and armature of difierential relay 55, and the contacts and amaturesof relays 99 and 78. When the called station disconnects, the diderential relay 55 will operate and re lease the line relay and duplex relay. and thereby release the connector switch l) restoring it to its normal condition. "When the calling subscriber disconnects the. printer equipment from the line E, difierential relay 55'operates and removes the ground connection through which'the switching relay 32 was held energized, Switch relay 32 is thereupon deenergized and opens at its contacts the circuits extending to the wipers 33, 34; and 35 and closes at its lowermost armature and contact the circuit of the release magnet 45. This circuit may be traced from grounded battery, winding of release magnet 45, 0d normalconta'ct 41, right-hand baclr contact and armature of relay 26, back contact and armature of relay 25 and the lowermost contact and armature of relay 32 to ound. Release magnet 45 is operated over t e circuit just traced and restores theselector switch C to normal. The cut-oil relay 16 is also deenergized at this time, whereupon contacts 18, 19 and 20 are open, due to the withdrawal ot the plunger 17. The system may now be used for the extension of the lines individual to the printing telegraph subscribers to other printing telegraph subscribers lines as may be desired.

Although the invention has been disclosed and described with reference to a particular circuit arrangement, it is of course obvious that various modifications may be made without departing from the spirit and scope ofthe present invention.

What is claimed is:

1. in a telegraph system, a plurality of telegraph ceivmg printer and transmitter, a two-way telegrap line, and selectively operable switching mechanism associated with each end of said line and under the control of said stations whereby connections may be extended in either direction over said line between said stations.

2. In a telegraph system, a'plurality of' telegraph stations each provided with a receiving printer and transmitter, a two-way tele raph line terminating at each end in a se ectively operable switch for extending connections from said line to a desired telegraph station, other selectively operable. switches for extending connections from said stations to said telegraph lines, and means associated with each telegraph station for controlling said switches whereby connec-. tions may be extended in either direction over said line between said stations.

3. In a telegraph system, a plurality of telegraph stations each provided with a re ceiving printer and transmitter and located at remote points, local telegraph lines for each .of said stations, telegraph hues equipped for duplex operation extending between said remote points, and selectively operable switches associated with each end of said last mentioned lines and under the control of said stations whereby connections may be extended in either direction over said duplex telegraph line between said local telegraph lines.

4. In a telegraph system, a plurality of telegraph stations each provided with a re ceiving printer and transmitter, a two-way telegraph line. switching means associated with each end of said line and under the control of said stations ,whereby connections may be extended in either direction over said line between said stations, and means operated following the. setting of said switching means for preventing its release on the normal operation of said telegraph transmitter.

5. In a telegraph system, a plurality of telegraph stations each provided with a receiving printer and transmitter. a telegraph line equipped for duplex operation. switching-means associated with each end of said line and under the control of said stations whereby connections may be extended in either direction over said line between said stations, and means operated following extension of said lines for automatically changing said system from full duplex to half duplex operation.

6. In a telegraph system, a plurality of telegraph stations each provided with a receiving printer and transmitter, a telegraph line, switching means under the control of any of said stations for extending connec- 'tions over said line to any desired one of said stations, and means operated when said line is found busy to eflect the transmission of characteristic impulses to the calling station whereby said receiving printer is operated to record the busy condition of said line.

ceiving printer and transmitter, a telegraph line and switching mechanism associated with each end of said. line and under the control of said stations whereby a connection may be extended in either direction over said line between said station, and relay means under the joint control of the connected station for effecting the release of said switching mechanism.

In testimony whereof, we have signed our names to this specification this 8th day of November, 1923.

GILBERT S. VERNAM.

DON ALD B. PERRY. 

